As one of the most comprehensive and objective methods to describe facial expressions, facial action coding system (FACS) has received more and more attention in recent years. It was originally proposed by Ekman in [1]} and expanded revised edition in [2]}. The revised version specifies 32 facial muscle movement units called Action Unit (AU). These motion descriptors describe the movements of the head, eyes, and other parts, such as cheek lift (AU6), eyebrow rise and fall (AU1, AU2), and lip bite (AU25, AU26). Human facial expression can not be simply described as the movement of some regions, especially when there are compound expressions and micro-expressions. Multiple local areas of the face will alternately or simultaneously appear slight changes in appearance, so it is difficult for classifiers trained based on discriminant methods to describe them. Any facial expression is the result of activation of a group of facial muscles, and each possible facial expression can be described as a combination of several AUs. Compared with the information judgment method, FACS can measure and evaluate facial movement in a quantitative way, so it is more objective and comprehensive.
